The present invention relates to an apparatus for detecting the permittivity of fuel supplied to a combustion chamber in a non-contact way to determine the properties of the fuel, and more particularly to an apparatus for measuring the alcohol content of alcohol-mixed fuel for use in automotive engines.
Fuel prepared by mixing alcohol with gasoline is being increasingly introduced for use in automobiles in order to reduce not only oil consumption but also atmospheric pollution due to automotive exhaust gas. However, the direct use of such alcohol-mixed fuel for engines designed to match with the air/fuel ratio of gasoline has made driving difficult because the theoretical air/fuel ratio of alcohol is lower than that of gasoline, that is, the air/fuel ratio of the former is leaned. Therefore, it has been customary to detect the alcohol content of alcohol-mixed fuel so as to regulate the air/fuel ratio, ignition timing and the like in accordance with the value detected.
In order to detect the percentage of alcohol, there have heretofore been proposed systems of utilizing the variation of electrostatic capacity to detect the permittivity of alcohol-mixed fuel. Among those of the sort stated above, Unexamined Japanese Patent Publication (Kokai) Sho-62-25248 (1987) and Post Examined Japanese Patent Publication (Kokoku) Sho-63-31734 (1988) disclose systems in which a coil is installed close to a fuel channel so that the permittivity of fuel is detected by utilizing the variation of floating capacity between the coil and another coil adjacent to the former or that of electrostatic capacity between the coil and an electrode adjacent thereto. Referring to FIGS. 1 to 3, such a system will be described.
FIG. 1 shows the construction of a conventional fuel permittivity detecting sensor, wherein numeral 1 denotes an insulating tube made of ceramic, oil-resistant plastic material or the like having a fuel passageway 4 inside, 16 an excitation electrode wound in a ring-like form on part of the outer periphery of the insulating tube 1, and 3 a single-layer winding coil also wound on the insulating tube 1 apart by a predetermined distance from the excitation electrode 16, these forming a sensor unit A. Further, reference B denotes a detection circuit connected to the sensor unit A and the following arrangement is made therein: the output of a sawtooth wave oscillation circuit 21 is connected to a voltage controlled oscillation circuit 22; the output of the voltage control oscillation circuit 22 is connected to the excitation electrode 16; one end of the single-layer winding coil 3 is grounded, this end being located opposite to the excitation electrode 16; a signal on the other end side of the single-layer winding coil 3 is connected via a full-wave rectifying circuit 23 to a peak detector 24; the output of the peak detector 24 is input to a sample hold circuit 25 to which the output of the sawtooth wave oscillation circuit 21 is input; and the output of the sample hold circuit 25 is output via a low-pass filter 26. FIG. 2A is a sectional diagram of the sensor unit A. FIG. 2B is a equivalent circuit diagram for the sensor unit. FIG. 3 is a graph showing the output characteristic of the sensor unit.
The operation of the aforementioned conventional sensor will subsequently be described. The frequency of the signal applied from the voltage control oscillation circuit 22 to the excitation electrode 16 is so controlled that it is swept by the output of the sawtooth wave oscillation circuit 21. If the permittivity .epsilon. of fuel is different from what is intended then, the induced voltage in the single-layer winding coil 3 will indicate a maximum value with a different frequency. This is because an electrostatic capacity Cf corresponding to the permittivity .epsilon. of the fuel within the fuel passageway 4 between the excitation electrode 16 and the single-layer winding coil 3 together with the self-inductance L of the single-layer winding coil 3 causes LC resonance, whereby the induced voltage of the single-layer winding coil 3 is maximized by the resonance frequency.
As shown in FIG. 2B, the series resonance frequency f.sub.0 of the equivalent circuit of the sensor unit is roughly expressed by EQU f.sub.0 =1/[2.pi..check mark.L.check mark.{Cf/(1+Cf/Cs)+Cp+Cpa}](1)
where L=self-inductance of the single-layer winding coil 3; Cf=capacity within the fuel channel 4 between the excitation electrode 16 and the single-layer winding coil 3, the capacity corresponding to permittivity .epsilon.; Cs=capacity of the tube wall of the insulating tube 1; Cp=capacity in the axial direction of the insulating tube 1 between the electrode 16 and the coil 3; Cpa=external floating capacity between the electrode 16 and the coil 3; and Cpc=floating capacity existing in parallel to the coil 3.
The resonance frequency f.sub.0 decreases as capacity Cf increase, that is, the permittivity .epsilon. of fuel increases as shown in Eq. (1). The induced voltage of the coil 3 is converted into a d.c. signal in the full-wave rectifying circuit 23 and its maximum value is detected by the peak detector 24. Further a peak detection pulse is supplied to the sample hold circuit 25 and the sweep output of the sawtooth wave oscillation circuit 21 is held for sampling. Therefore, the holding voltage at this time becomes equivalent to the resonance frequency f.sub.0 and the voltage output is output outside via the low-pass filter 26 as Vout.
The following problems inevitably arise as the electrode 16 is coaxially arranged on the edge face of the coil in the aforementioned conventional sensor. For instance, given L=20 .mu.H; the outer diameter .phi. of the insulating tube 1=10 mm; the wall thickness t of the insulating tube 1=1 mm; and the distance d between the edge face of the coil 3 and that of the electrode 16=2 mm, large parallel capacities Cp, Cpa with respect to the capacity Cf varying with the permittivity .epsilon. of fuel exist when the fuel is a mixture of gasoline of .epsilon.=2 and methanol of .epsilon.=33 and since their resonance frequencies f.sub.0 equally become about 8 MHz, the difference therebetween only remains at about 5% as shown in FIG. 3.
Due to the distance d between the edge faces of the coil and the electrode, moreover, there arise some problems including the difficulty of securing precision, the wide variation of resonance frequency f.sub.0 depending on the surface condition changing with the dirt in the insulating tube 1, the external moisture or the like, and these worsen output reproducibility. If, the distance d is increased to secure precision, the capacity Cf also decreases, though the capacities Cp, Cpa are reduced and the permittivity of f.sub.0, far from abating, tends to decrease while the resonance frequency f.sub.0 only increase on the average. In other words, it is not possible to secure a high output changing ratio relative to the change of the permittivity .epsilon. in the conventional sensor and a great sensor-to-sensor output variation causes the sensor output to be easily affected by the external condition. The problem is that the sensor precision is poor.
There arises a further problem in that since the permittivity of fuel has temperature characteristics, the resonance frequency tends to vary, depending on the temperature measured even though the fuel has the same concentration.
A temperature compensation method is generally implemented by providing a thermistor within a fuel channel and connecting the thermistor to a temperature compensation circuit. However, such an arrangement not only increases the size of the apparatus but also makes it costly because a new circuit is additionally installed.